1. Field of the Invention
The present invention relates to a data transmission system, and more particularly to a data transmission system in which cell retransmission can be avoided when a packet is divided into a plurality of cells in an ATM adaptation layer (AAL) of an ATM (asynchronous transfer mode) communications system.
2. Description of the Related Art
The technique on the loss detection control system of an ATM cell is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 8-186571: a first conventional example). In this loss detection control system, when an EOP (End Of Packet) cell which is the last cell does not arrive for a predetermined time of period, the EOP cell is determined to have been lost. Thus, the cell loss can be detected in a shorter time period, compared with the case where cell loss is detected by an upper level application. Also, the invalid suspension by a buffer memory for the assembly of cells can be prevented.
Also, the technique on a cell discard control system is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 4-372245: a second conventional example). In this cell discard control system, when a cell of a data packet is discarded, the discard of the other cells of the data packet is variably controlled, considering the service class. Thus, efficient communication is made possible in accordance with an upper level application.
However, in the above-mentioned conventional examples, only the case which the discard of a cell has occurred is considered. It is not solved at all to reduce the possibility of the discard of cell. This is because the problems such as the short detection time of cell discard and the influence to the upper level layer due to the cell discard are only considering.
In addition, a cell retransmission control system is described in Japanese Laid Open Patent Disclosure (JP-A-Heisei 6-121001). In the cell retransmission control system, a procedure of cell retransmission in an AAL (ATM Adaptation Layer) is simplified to avoid retransmission of an unnecessary cell.
In Japanese Laid Open Patent Disclosure (JP-A-Heisei 7-250082), there is described an AAL type 1 processing unit in which any delay time is not generated even if a phase proceeding is generated due to switching and underflow is not generated even if a phase delay is generated due to the switching.
Also, in Japanese Laid Open Patent Disclosure (JP-A-Heisei 8-116326), there is described a cell assembling unit in which a delay buffer section outputs a cell to a payload accumulating section with a delay time during which the cell is analyzed such that the cell assembling unit can be constructed with a small size.
Also, in Japanese Laid Open Patent Disclosure (JP-A-Heisei 7-87099), an error control method is described in which reliable data communication is made possible with high through-put and low latency in an AAL type 5.
Also, in Japanese Laid Open Patent Disclosure (JP-A-Heisei 8-46629), an ATM network interface module is described in which throughput in an upper layer can be improved by adding hardware of as a small quantity as possible.
Also, in Japanese Laid Open Patent Disclosure (JP-A-Heisei 8-237282), a congestion control system in a packet communication system is described in which data such as the number of accumulated ATM cells and the number of discarded ATM cells are monitored to see a buffer state and whether an ATM cell should be discarded is determined based on such data.